The threat of impact on Earth of an asteroid or comet,
while of very low probability, has the potential to create public panic and --
should an impact happen -- be sufficiently destructive (perhaps on a global
scale) that an integrated approach to the science, technology, and public
policy aspects of the impact hazard is warranted.This report outlines the breadth of the issues that need to be
addressed, in an integrated way, in order for society to deal with the impact
hazard responsibly.At the present time,
the hazard is often treated -- if treated at all -- in a haphazard and
unbalanced way.

Most analysis so far has emphasized telescopic searches
for large (>1 km diameter) near-Earth asteroids and space-operations
approaches to deflecting any such body that threatens to impact.Comparatively little attention has been
given to other essential elements of addressing and mitigating this
hazard.For example, no formal linkages
exist between the astronomers who would announce discovery of a threatening asteroid
and the several national (civilian or military) agencies that might undertake
deflection.Beyond that, comparatively
little attention has been devoted to finding or dealing with other potential
impactors, including asteroids smaller than 1 km or long-period comets.And essentially no analysis has been done of
how to mitigate other repercussions from predictions of impacts (civil panic),
how to plan for other kinds of mitigation besides deflection (e.g. evacuation
of ground zero, storing up food in the case of a worldwide breakdown of
agriculture, etc.), or how to coordinate responses to impact predictions among
agencies within a single nation or among nations.

We outline the nature of the impact hazard and the
existing ways that a predicted impact would be handled at the present
time.We describe potential solutions
to existing gaps in the required approaches and structures (both technical and
governmental) for dealing with impacts, including the kinds of communications
links that need to be established and responsibilities assigned.

We recommend crafting, adoption, and implementation of
improved procedures for informing the broader society about the impact hazard,
notifying the public and relevant officials/agencies about an impact
prediction, and putting in place (in advance of such predictions) procedures
for coordination among relevant agencies and countries.We recommend that pro-active steps be taken,
perhaps through a high-visibility international conference and other types of
communication, to educate the broader technical community and public policy
makers about the impact hazard and the special aspects of mitigating this
atypical hazard.For example, the most
likely international disaster that would result from an impact is an unprecedentedly
large tsunami; yet those entities and individuals responsible for warning, or
heeding warnings, about tsunamis are generally unaware of impact-induced
tsunamis.We also recommend that
additional attention be given to certain technical features of the hazard that
have not received priority so far, including the need to discover and plan
mitigation for asteroids smaller than 1 km and for comets, study of the
potential use of space-based technologies for detection of some kinds of
Near-Earth Objects, study of chemical rockets as an approach to deflection that
is intermediate between bombs and low-thrust propulsion, and further evaluation
of the risks of disruption (rather than intended deflection) of an oncoming
object.

Finally, we believe that international human society (and
elements of it, like the U.S. government) needs to make an informed, formal
judgement about the seriousness of the impact hazard and the degree to which
resources should be spent toward taking steps to address, and plan for
mitigation of, potential cosmic impacts.The existing unbalanced, haphazard responses to the impact hazard
represent an implicit judgement; but that judgement does not responsibly
address the extraordinary and unusual consequences to nations, or even
civilization, that could result from leaving this hazard unaddressed in such an
arbitrary, off-hand way.For example,
we believe it is appropriate, in the United States, that the National Research
Council develop a technical assessment of the impact hazard that could serve as
a basis for developing a broader consensus among the public, policy officials,
and governmental agencies about how to proceed.The dinosaurs could not evaluate and mitigate the natural forces
that exterminated them, but human beings have the intelligence to do so.

{This SwRI White Paper is also available at:www.boulder.swri.edu/clark/neowp.html}

INTRODUCTION

The impact hazard from near-Earth asteroids and comets
has evolved from a science fiction scenario to a serious societal issue during
the past twenty-five years.The
scientific community began to understand the implications for life on Earth of
errant small bodies in the inner solar system in 1980 when Nobel Laureate Luis
Alvarez and his colleagues published an epochal paper in Science
(Alvarez et al. 1980) advocating asteroid impact as the cause of the
great mass extinction 65 million years ago that led to the proliferation of
mammal species.The same year, the NASA
Advisory Council advocated study of a modern-day cosmic threat to civilization,
leading to a formal study (The Spacewatch Workshop, chaired by Eugene
Shoemaker) the following year.

A decade later, these scientific issues first received
significant public consideration when lobbying efforts by the American
Institute of Aeronautics and Astronautics (AIAA) and others resulted in action
by the U.S. House of Representatives, which directed NASA to study the impact
hazard.NASA responded by organizing an
International Conference on Near-Earth Asteroids and two study workshops, one
(chaired by David Morrison) leading to recommendations (Morrison 1992) for a
telescopic "Spaceguard Survey" of the larger Near Earth Asteroids
(NEAs) and the second (chaired by John Rather) evaluating a host of potential
approaches to mitigation of an impending hazard should an asteroid be found to
be on a collision course with Earth (Rather et al. 1992).

During the 1990s, numerous scientific and engineering
conferences have been held worldwide concerning the impact hazard (including
one held at United Nations headquarters, Remo 1997) and public interest groups
were established in several nations, mostly associated with the Spaceguard
Foundation (http://spaceguard.ias.rm.cnr.it/SGF/).Despite official notice being taken by several national and
international entities (e.g. the Council of Europe), little serious attention
has yet been given by governments to evaluation of the NEO hazard or
preparations for dealing it (NEO = Near Earth Objects, including comets in
addition to NEAs).NASA, in collaboration
with the U.S. Air Force, is the major supporter of NEO research, with a few
million dollars per year devoted almost solely to the use of existing
telescopes to search for, and find by 2008, 90% of the NEAs larger than 1 km
diameter (http://neo.jpl.nasa.gov).In
late 2000, a task force recommend­ed that the British government consider
taking initial steps to support efforts to research the impact hazard (Atkinson
2000; http://www.nearearthobjects.co.uk/index.cfm); in late February 2001,
however, the government responded not with concrete action but only promising
to study the matter further and formulate an international approach to the
issue.

Other major elements of the impact hazard remain
unaddressed.Searches for comets and
for smaller NEAs are in their infancy.And little or no serious, official actions have been taken by
governments to be prepared to respond to any announcement of a specific impact
threatened in the years or decades ahead.For example, Dr. Brian Marsden, who directs the International
Astronomical Union's Minor Planet Center (where most astronomical data
concerning NEOs is cataloged: http://cfa-www.harvard.edu/cfa/ps/mpc.html),
recently said that he had no idea who in the United States government would be
receptive to serious information he might have one day about an impending
impact.Surely some agencies would
be interested, but communication pathways, responsibilities, and implementation
plans have yet to be established.

This White Paper has been supported primarily by a
Presidential Discretionary Internal Research and Development grant from
Southwest Research Institute.Its
purpose is to outline elements of a systematic approach, with various options,
for dealing with the full breadth of the impact hazard -- starting with issues
about discovery of potentially dangerous bodies, proceeding through societal
issues about evaluating the hazard and taking appropriate advance measures, to
actual mitigation of potentially threatening impactors.We conclude with some recommendations that
might lead to a more comprehensive and balanced approach for
twenty-first-century society to take toward a very real, if low probability,
threat that could conceivably doom everyone we know and everything we care
about.

DETECTION AND IMPACT
EFFECTS

Detection

One major phase of impact hazard research is to identify
and characterize potentially hazardous bodies and, in particular, to find any
impactor long enough before it hits so that mitigation steps may be taken.This phase of research involves preliminary
characteriza­tion of a potential impactor (perhaps even before there is any
conclusive prediction that it will hit), as well as statistical
characterization of physical properties of NEOs generally so that any future
identified impactor may be quickly placed into an understood context.We address more detailed characteriza­tion
of such a body under "Mitigation" below.

90% of Large NEAs.Current observational approaches to
cataloging potentially threatening NEOs emphasize use of dedicated groundbased
telescopes equipped with CCD detectors and automated routines for identifying
possible NEOs.While telescopic search
programs are underway, or gearing up, in several countries (though one program
was stopped several years ago in Australia), most are located in the United
States and are coordinated through the NASA NEO Program Office at the Jet
Propulsion Laboratory (JPL).NASA's
stated goal (Pilcher 1998) is to find 90% of the larger NEAs (>1 km
diameter) by 2008.These are asteroids
large enough to potentially cause a global climate disaster and threaten the
continuation of human civilization (Chapman & Morrison 1994).The majority of such discoveries (~75%) are
currently being made by the LINEAR project (www.ll.mit.edu/LINEAR/), operated
by M.I.T. Lincoln Laboratory primarily with U.S. Air Force funding.The LONEOS program at Lowell Observatory is
currently running a distant second, and other search efforts trail LONEOS.

It is estimated (D. Morrison, NEO News, 20 January 2001)
that ~50% of the larger NEAs have been found, depending on how many there are
>1 km diameter (current estimates range from 700 to 1100).Since the more difficult-to-detect NEAs may
take longer to find, it is not yet clear whether projected search programs will
reach the 90% goal by 2008.It is
nearly too late to begin building new telescopes in time to contribute
meaningfully to meeting the deadline.An ancillary concern is whether the astrometric follow-up capabilities
(largely undertaken by amateur astronomers and poorly-funded professionals in
several nations) will ensure that reliable orbits can be established for newly
found NEOs so that they will not be lost.Another ancillary issue is that physical observations of the discovered
NEAs -- for purposes of determining their sizes, shapes, spin rates, and
composition -- are proceeding very slowly.These are necessary for two reasons: (a) determination of size is
required to ensure that discovery of NEOs of particular brightnesses actually are
1 km in diameter or larger; (b) an assessment of physical properties is
ultimately the first step in developing a mitigation approach (see below).

Once 90% of large NEAs have been cataloged and found to
be "safe" for the foreseeable future (as is very likely but not
assured), then the risk to civilization will be known to be a factor of several
(perhaps approaching an order-of-magnitude) lower than it was a decade ago when
few NEAs had been found.However,
several categories of potentially threatening NEOs remain mostly unaddressed by
concentration on these larger asteroids:the remaining 10%, smaller NEAs, and long-period comets.

Last 10% of Large NEAs.The remaining NEAs >1 km diameter will
include some that were just, by chance, missed.(During the last year, some surprisingly bright NEAs were
discovered that somehow had escaped three decades of surveillance that would
have been expected to have found them.)But many of the remaining NEAs will preferentially be those that are
difficult to discover.Attributes of
hard-to-find NEAs include very low albedo, location in orbits with periods
commensurable with cloudy seasons where the chief search telescopes are
located, those with very high inclinations, etc.

Perhaps the most difficult-to-find NEAs are Atens (NEAs
with semi-major axes <1 AU [1 AU = mean distance between Earth and Sun] but
which cross the Earth's orbit and are seen for brief intervals at large angles
from the Sun) and objects that orbit wholly within the Earth's orbit (but may
come nearly tangential to the Earth's orbit). Historically, most search programs have concentrated on the
"opposition point," opposite the Sun in the sky, where NEAs are
brightest (like the full Moon).In the
last couple of years, this has been changing, and some search programs are now
trying to search specifically for Atens.But appreciable inefficiencies exist in such groundbased efforts (often
such NEAs are in a dark sky and above the horizon for short durations or are
wholly lost in the Sun's glare).So the
question is raised about whether these objects -- and their smaller cousins --
might be better hunted for from locations in space, such as near the orbit of
Venus where these bodies will be more fully illuminated by the Sun and are
observable against a black sky away from the Sun for most of the time.

NEAs Smaller than 1 km.While NEAs <1 km diameter probably cannot
threaten a global catastrophe, those between 200 m and 1 km in diameter also
pose a significant threat.One could
cause a tsunami of size and destructive power perhaps unprecedented in
historical times, threatening everything near the coast of whatever ocean
happens to be struck.Moreover, such
objects are much more numerous than the larger ones and hence they strike Earth
perhaps tens of times more frequently than the civilization-threatening
asteroids.That means that there is a
much higher chance (perhaps 1%) that we or our grandchildren will actually have
to deal with such a disaster during this century.

While smaller NEAs are being found in great numbers
(roughly twice as many have been found as those >1 km diameter), it would be
very challenging to undertake a nearly-complete census of them, as is being
done for the larger NEAs by Spaceguard.200 m NEAs are 3 magnitudes fainter than 1 km asteroids.Suitable, dedicated groundbased telescopes
(large [roughly 5 m] aperture, sensitive detectors) would have to be
constructed in appropriate numbers.A
recent recommendation by the American astronomical community (National Research
Council 2001) is for construction of a single 8.4 m telescope, the
Large-aperture Synoptic Survey Telescope (LSST), which would have as one of its
major objectives, cataloging 90% of NEAs > 300 m diameter in one
decade.There are difficulties in
searching for small asteroids, such as problems with background objects (stars
and galaxies) and the night sky background; infrared techniques could address
some of the problems.The required
astrometric follow-up programs would have to be developed as a separate, professional
effort since the vast majority of these NEAs would be beyond the capabilities
of the volunteers currently following up brighter discoveries.Physical characterization of small NEAs is,
of course, correspondingly more difficult than for the larger ones.

Long-period comets.Comets could be a significant part of the impact hazard, with estimates
ranging from a few percent to nearly half the problem.And they pose great difficulties for both
discovery and mitigation.Comets, until
they "turn on", are many magnitudes fainter than comparable asteroids
(because of greater distance and sometimes lower albedo); if they are to be
discovered before reaching Jupiter's distance from the Sun, a survey might have
to go 7 magnitudes fainter than Spaceguard.To discover incoming kilometer-scale comets before they reach Jupiter's
distance (and even that might give us inadequate warning -- see below), a rough
estimate is that we would need to deploy thirty 10 m aperture telescopes at
Earth or 50 to 100 2.5 m telescopes at Jupiter's distance.Such extravagant projects seem far beyond
the realm of practicality, so innovation will be necessary to begin to address
the comet hazard.

Search techniques.There are three basic approaches to searching for potentially
threatening objects: groundbased, spacebased near-Earth, and
interplanetary.Groundbased searches
have the enormous cost advantage of not having to be launched into space and
maintained there.However, the duty cycle
is restricted by daylight and cloudy weather, and the atmosphere degrades sensitivity
in several ways.Earth-orbital
observatories overcome some of these difficulties and might be of comparable
cost if they could be piggy-backed onto some of the many other enterprises that
operate Earth-orbiting satellites.Recently, there was reported to be a potential opportunity -- for little
more than the cost of building the instrument -- to fly an NEA detector on a
Canadian satellite (A. Hildebrand, 2000, personal communication).Interplanetary observatories are the most
costly of all, but have the potential for enormous gains over Earthbased
techniques in detectability of certain kinds of NEAs.Such gains are much more likely to be realized for objects like
Atens, which are confined to the comparatively modest volume of the inner solar
system and could be searched for advantageously from a location well inside the
Earth's orbit.The advantage of getting
closer to outer solar system objects, like comets, is overwhelmed by the
enormous volume of space that would have to be searched from a location like
Jupiter's orbit.

Search techniques currently operate at visible
wavelengths, where the reflected sunlight is brightest and detectors are
exceptionally efficient.However,
background problems may be overcome by looking at other wavelengths where NEAs
have signatures atypical of stellar objects (e.g. thermal infrared; cf. Tedesco
et al. 2000).Radar is
impractical for searching for objects of the sizes that pose regional or global
danger, which need to be found long before striking Earth, generally at great
distances.

Impact Effects

Little specific research has been funded to investigate
the potential environmental and societal consequences of impact.Most of what is known has been derived (for
smaller impacts) from extrapolations from nuclear weapons tests (cf. Glasstone
& Dolan 1977), from numerical simulations, and (for larger impacts) from
inferences from the geological record for the Cretaceous/Tertiary (K/T) impact
65 Myr ago.In addition, some
consequences are analogous to effects studied in the 1980s in the context of
Nuclear Winter.Global circulation and
climate models have been used to simulate atmospheric perturbations due to dust
and aerosols lofted by impacts.Larger
impacts, especially, have diverse physical, chemical, and biological
consequences, which dominate the fragile ecosphere of our planet and may be
expected to act in synergistic ways that are difficult to imagine and
model.Therefore, there is considerable
uncertainty about the environmental consequences of larger impacts.The greatest danger from smaller impacts
(impactors several hundred meters in diameter) are tsunamis, which very
efficiently transfer the effects of a localized ocean impact into dangerous,
breaking "tidal waves" on distant shores (Ward & Asphaug 2000).

Table 1 describes some of the more common immediate
environmental consequences from impacts by NEAs in three size ranges:regional disasters due to impacts of
multi-hundred meter objects that impact Earth every 104 years;
civilization-ending impacts by multi-km objects that occur on a million-year
timescale; and K/T-like cataclysms that yield mass extinctions on a 100 Myr
timescale.Most of the listed
consequences are derived from the recent review by Toon et al. (1997).Naturally, individual events may vary due to
factors like angle-of-attack (oblique impacts are generally much more damaging
than vertical impacts by the same size bodies), whether the impact is into land
or ocean, and even the geology of the target region (e.g. it has been
hypothesized that the K/T impact may have yielded especially large quantities
of aerosols because the target region was rich in anhydrite).However, to first order, the consequences of
impacts are simply in proportion to their explosive energy:impacts generate gigantic explosions by the
virtually instantaneous conversion of the enormous kinetic energy of the
asteroid or comet into other highly destructive forms of energy.

Impacts that are even smaller and more frequent impacts
than those shown in Table 1 -- like the 15 Megaton impact in Tunguska, Siberia,
in 1908 -- may have major consequences near ground zero.But other natural disasters, like
earthquakes and floods, having the same damage potential (e.g. human
fatalities), happen at least a hundred times more frequently than small
impacts.Perhaps the most serious
consequences of impacts similar to and smaller than Tunguska, which happen on
timescales comparable to or shorter than a human lifetime, are unpredictable
reactions by observers.A bolide ten
times brighter than the Sun occurred in the Yukon in January 2000, yielding
some meteorites.Such an event in an
unstable location in the world could be misinterpreted as an enemy attack and
precipitate war.Another possibility is
that a small impact could generate political ramifications and fallout from the
public, knowledgeable to some extent that NEA searches and mitigation efforts
are underway, and angered at those who were 'supposed' to be on guard for such
events (W. J. Cooke 2000, personal communication).

There has been essentially no modelling at all of the
possible economic and social consequences of the kinds of environmental damage
listed in Table 1.Clearly, in cases of
impactors >1 km in diameter, we enter a realm never previously encountered
by modern civilization.Even the great
World Wars of the twentieth century left many nations relatively undamaged, and
they were thus able to serve as nuclei for recovery.An unexpected impact by a 2 km asteroid might well destroy
agriculture in both hemispheres and around the world, leading to mass
starvation from which no nation would be immune.Impacts may also precipitate catastrophic failures of modern
communications and power infrastructures.Possible mass psychological reactions to such a devastating catastrophe,
while portrayed in science fiction novels and movies, have also not been
researched in an impact hazard context.Even a near-miss by a dusty comet could have serious ramifications,
without even impacting: loss of many satellites in the geosynchronous
constellation due to dust impacts and associated plasma arcing could severely
disrupt global communications and associated economic and security
infrastructures (P. Brown 2000, private communication).

EVALUATION AND WARNING

Recent history has provided several cases in which
astronomers discovered potentially dangerous asteroids and announced finite
probabilities that they would crash into the Earth within the next few decades
(cf. Chapman 2000, Chapman 2001).Even
a very low probability of a devastating impact occurring on a precise day in
our personal future lifetime generates great interest, news coverage, and
fear.Given the unlikelihood of an
actual catastrophic impact, the most likely realities that society may have to
face are misunderstandings about such impact predictions, close approaches of
NEOs, and actual small impacts that may be misinterpreted.Within the last few years, the astronomical
community has made some progress in understanding and improving its own role,
while there remains little awareness on the part of governmental entities about
how to respond to potential NEO events.

Existing Structure

Formal approaches to handling NEO observations and
orbital calculations, in a hazard context, are rather minimal but are more
developed than they were prior to the March 1998 prediction, and subsequent
retraction, of a close approach and possible impact by 1997 XF11.The nucleus of activity remains the Minor
Planets Center (MPC) of the International Astronomical Union (IAU), located in
Cambridge, Massachusetts.Through long
tradition, observers (including the major telescopic NEO search programs that
comprise Spaceguard) continue to send observations of NEO positions to the
MPC.These positions are cataloged and,
in an increasingly thorough and routine way, are used to calculate orbits and
search for potential future impacts.Many of the data are made available to the larger astronomical
community, and efforts are underway at several institutions (including
University of Helsinki, Lowell Observatory, the University of Pisa, and Jet
Propulsion Laboratory) to make independent orbital calculations and
predictions.

Through a protocol developed at a 1999 workshop in
Torino, Italy, and subsequently adopted as de facto procedure by the IAU's
Working Group on NEOs (WGNEO), astronomers decide if a future impact prediction
is sufficiently important to warrant a careful independent check before public
announcement.(There is an intention to
be open, but it is recognized that errors and cries of "Wolf!" are
much more likely to happen than actual impacts, so the desire is to weed out
the mistakes before making an official public announcement.)If a predicted impact meets certain criteria
(based on impact probability and size of impactor, as codified in the Torino
Impact Hazard Scale [Binzel, 2000]), then a 72-hour process begins of
peer-review by a subcommittee of the WGNEO.After that process is concluded, the IAU may post on its web-site an
official confirmation that the prediction has been checked.

Because of a failure (at least in the public relations
sense) of the system that happened in autumn 2000 (regarding the small object
2000 SG344), the 72-hour period may be changed in the near future and there is
increasing emphasis on the need to avoid secrecy in the future.The hope is that the news media will wait,
before publicizing a predicted impact possibility, for official confirmation
that the prediction is valid, without astronomers being required to keep all
information about the matter secret.

Various journalistic protocols, rapidly changing in the
age of the Internet, then govern the public dissemination of information about
potential impacts.In the past, various
entities (including, for example, the Press Officer of the American
Astronomical Society) have facilitated dissemination of news about NEOs, but no
formal procedures exist.Neither do any
formal procedures exist, that we know of, for information about potential
impacts to be considered or acted upon by national or international governments
or other entities.Informally, NASA's
NEO Program Office at JPL is kept "in the loop" and its personnel
are, no doubt, expected to report matters of importance up the chain of command
in NASA.Similar organizational
reporting procedures presumably operate within other entities that are likely
to discover NEOs or witness an actual impact, like the U.S. Air Force Space
Command, which operates satellites that detect major bolides in the Earth's
atmosphere.However, there surely is an
early disconnect in formal procedures when the IAU confirms an impact
prediction by a posting on its web page.

Potential Structure

Given the widespread public interest in the impact hazard
and the potential seriousness of an impact, there ought to be formal procedures
for evaluating information about potential impacts, an outline of what should
be done in various cases, and assignment of responsibilities to relevant
agencies.Evaluation of potential
impact predictions needs to go beyond the verification of orbital calculations,
which is what the IAU WGNEO's peer-review process currently focusses on.The 2000 SG344 event showed that there was
an unexpectedly great uncertainty in estimating the size, hence potential
dangerousness, of the potential impactor -- it could have been quite serious
(Tunguska-scale) or quite innocuous (if the object is a wayward booster rocket
rather than an asteroid).Hazard
evaluation and responsibility for making public announcements should be an
activity expanded far beyond astronomers to the broader civilian and military
communities of experts with experience in dealing with natural hazards and
disasters.

We briefly consider this topic from both a U.S.-national
and an international perspective.

U.S. National.Initial
information about potential impacts will likely be developed within NASA or the
Air Force.NASA's NEO Program Office
has responsibility within NASA, but it is a very small operation and is in the
infancy of developing communications and response protocols for NASA as a
whole.The Air Force Space Command and
the Air Force-sponsored LINEAR search project (operated by M.I.T. Lincoln
Laboratory) would be the first to handle NEO information obtained from Air
Force programs.Other entities may have
much more experience in dealing with analogous issues.For example, NOAA's Space Environment
Center, in Boulder, Colorado, forecasts space weather (e.g. solar storms that
may disturb the Earth's geomagnetic environment) and operates jointly with the
Air Force the Space Weather Operations (SWO), which is the national and world
warning center for space disturbances that can affect operations in space.

NEOs could be added to the SEC's duties.Alternatively, its joint operation of the
SWO might serve as a model for creation of an interagency center that would
deal with NEO issues.At the moment,
there is no movement toward advancing beyond the ad hoc procedures
currently in place.The next step would
be to organize relevant national agencies that would be involved in mitigating
an actual impact or dealing with issues arising from misunderstandings,
"near-misses," etc.As a
matter of policy, NASA has so far declined to accept any responsibility for
mitigation.Other entities (many of
them ultimately under the aegis of the National Security Advisor) that might be
involved in consideration of preparation for mitigation include the Air Force,
the Department of Energy (much interest and expertise in asteroid deflection,
for example, already exists at Los Alamos, Sandia, and Livermore National
Laboratories), the Federal Emergency Management Agency (FEMA), and tsunami
warning entities (which are most thoroughly developed for the Pacific Rim
region).

International.The
United Nations Office for Outer Space Affairs has undertaken several activities
(including sponsorship of some meetings and investigation of building a search
telescope in Africa) but has not, to our knowledge, developed an approach for
formal U.N. response to a potential crisis.Current efforts of the IAU (described above) might ultimately be
combined with other scientific interests in the NEO hazard through the
International Council for Science (ICSU), of which IAU is part.The ICSU has partnership relations with
other relative entities (including the Council of Europe, which is formally on
record as supporting NEO research, and elements of the United Nations, but not
formally the U.N. Office for Outer Space Affairs).

Several countries support modest NEO observing programs
and several governments with space programs have made statements about the NEO
threat.The most substantive
considerations at the moment are in the United Kingdom, where the government
has recently responded to recommendations for addressing NEOs presented by a
commission at the request of Parliament (Atkinson, 2000); while not committing
to funding concrete actions (like building a new search telescope or establishing
the recommended "British Centre for NEOs"), the government did
promise to work within ESA toward coordinating substantive steps within
Europe.International coordination of
practical responses to a potentially hazardous impact might be initiated
through the Inter-Agency Consultative Group for Space Sciences (IACG), which is
the forum for interaction between NASA (USA), ESA (Europe), ISAS (Japan), and
RASA (Russia).However, we are aware of
no discussions about the NEO hazard within that forum.

Another international forum for discussion and
coordination of NEO research is the an NGO called the Spaceguard Foundation
(SGF), which is headquartered in Italy.For example, under sponsorship of ESA's European Space Operations Centre
(ESOC), the SGF undertook a study of both groundbased and spacebased systems
for identifying and characterizing NEOs (Carusi 2000).National chapters of the SGF exist in
several countries.

Detection/Deflection/Mitigation.Most discussion of the NEO hazard has so far
centered on technical discussion of systems for (a) detection of a potential
impactor and (b) deflection of an oncoming body by means of space
operations.An outline of how
observations may be vetted among all relevant players and evaluated for
potential mitigation steps has not yet been developed.We suggest that one or more of the national
and international entities described above consider developing recommendations
for the proper procedures that should be followed, once the IAU process
identifies and confirms a potential future event.

A better understanding needs to be developed about how
such information is disseminated to the public and to public officials (both
through the unfettered activities of the news media and by official actions of
government entities).Decision-making
forums need to be identified in advance of a crisis, both national and
international.While it is very
unlikely that an actual impact will require urgent response (it is ten times
more likely that an impact will be identified that is decades away rather than
just years away), the public response to an identified future event may well
require immediate action.

For starters, the IAU WGNEO procedures need to be
critically evaluated.Is the 72-hour
review time appropriate?Should the duration
for review depend on the time until predicted impact?We suggest that an appropriate review period might be one day per
year-to-impact, not to exceed two weeks.How much in the open should such reviews be conducted?How much urgency should be given to requests
for confirmatory searches of past archives or new observations of a potentially
hazardous object by existing telescopes?Beyond posting a confirmed prediction on its Web site, should the IAU
actively inform other entities; if so, which ones?(At the moment, there is no formal way that predictions are sent
up the chains-of-command.)And, then,
which entities (in the United States, in the international arena) should be
charged with formal responsibility for evaluating how to handle public evaluation
of the potential crisis and implementing mitigation procedures?As a concrete example, from whom should high
FEMA officials expect to receive reliable information that might cause them to
set on-the-ground mitigation in motion?

Social/Political/Economic/Technological Issues.Numerous issues need to be addressed in
appropriate forums in order to derive a consensus on how to address the wider
NEO threat...beyond the narrow technical astronomical/space operations arenas
in which most discussion has so far taken place.

lWhat is the appropriate level of
public/governmental attention to the NEO Hazard that should be given?Should more telescopes be built?Should significant efforts be made by
agencies that have so far given little or no attention to the NEO hazard, which
necessarily might require a modest shift of emphasis from their current
efforts?Has the NEO hazard actually
been overblown by the naturally heightened public response engendered by
Hollywood hype?How have the views of
the public and of public officials been distorted by the "cries of
'Wolf!'" which have occurred during the last few years?

lOne element of addressing the question of
how much attention, and how many resources, should be devoted to the NEO hazard
would be an objective assessment of the anticipated economic and actuarial
consequences of this hazard in the context of other natural hazards, diseases,
and environmental concerns that compete for funds.

lThere are strong political obstacles, in an
era when governments are curtailing expenditures and considering massive tax
cuts, to starting new programs.This may be one reason for footdragging by entities, like NASA, that
might have otherwise been expected to adopt a more aggressive stance toward
dealing with this hazard (especially since NASA's Congressional oversight
committee has several times asked it to do so).What are other reasons for the disinclination to do more about
the NEO hazard, and are they valid or should they be overcome?

lThere have already been instances of
interagency squabbling over who should take responsibility for projects related
to the NEO hazard (e.g. creation of the cooperative arrangements between the
Air Force and NASA over the Clementine mission were not smooth).How will responsibilities be assigned in a
cooperative, non-redundant way, both nationally and internationally?

lWhat are the relative national and
international responsibilities?In
theory, until an impactor is discovered, all nations are at risk.A large enough impact would necessarily have
global effects, and even a much more modest impact into the ocean might affect
many nations.But by far the most
likely actual impact in the near future (if one occurs at all) will be one of
modest size that will have only local consequences, probably in a single
nation.

lProcrastination is a serious issue.Already a dozen years have passed since
major public discussion began about the NEO impact hazard, and some think that
the failure of politicians to act has to do with the fact that an impact -- or
the need to mitigate one -- is extremely unlikely to happen on a current
politician's "watch".History
is rife with cases where steps were put off into the future for dealing with
seemingly unlikely disasters (perceived as going to happen in the distant
future) like hundred-year floods.How
do we ensure that the NEO hazard is dealt with soon enough in order that mitigation
is reliably effective?What is an
appropriate response to the claim that future technology will be much more
capable of dealing with the hazard (e.g. deflecting an oncoming asteroid) than
current technology, so we should wait until that technology has been invented
and developed inasmuch as no imminent impact is known or likely to happen?

lCertain elements of the impact hazard can
already be seen to be extremely difficult to address.Much of the hazard associated with NEAs >1 km diameter will be
addressed within the next decade: most will be found by the telescopic search
programs and we already have cogent ideas about how to investigate and deflect
such NEAs.However, as discussed below,
comets present a much more serious issue: it is unlikely that we will have
sufficient warning to practically address the catastrophe threatened by an
oncoming comet in time.Advance
preparations of several sorts might help or give us a better chance to save
ourselves.What are the advantages and
disadvantages of taking preparatory steps to mitigate a comet impact, in
advance of discovering that one is bearing down on us?For example, Carl Sagan argued that building
deflection technologies in advance might be more dangerous than the threat they
are designed to address (Ahrens & Harris 1994).

lShould response to the NEO hazard be
considered "civil defense" to be undertaken by civilian agencies
(e.g. FEMA in the United States) or as "planetary defense" to be
undertaken by national or international military forces?

lWe have little idea about the role that a
civilian agency like FEMA might play in the NEO hazard (it has so far given
essentially zero consideration to the issue at all).We know even less about the analogous entities in other countries
as well as international entities that similarly need to be informed about this
issue.

MITIGATION

Characterization of the
Threatening Object

What we need to learn.The earliest information we will have,
beyond knowing the orbit that determines that it will approach Earth, is a hint
about the object's size (based on its brightness, which will specify its level
of hazard on the Torino Scale).This
might quickly be augmented by other data, if physical observations are obtained
during the same apparition (more accurate size from radiometry and photometry,
some information on shape and spin period from lightcurve photometry, and
spectrophotometric indications of taxonomic association and mineralogy).In order to more accurately assess the
danger and practicalities of mitigation, the most important additional
information needed is:

How do we learn it?Earth-based observations with the largest, most powerful telescopes and
radars can provide improved estimates of size, shape, spin-state, and
composition.These instruments could be
employed rapidly, in most circumstances, if needed.Major advances in characterizing an asteroid require at least a
spacecraft flyby.Unless the warning is
very short or the threat is deemed to be marginal, one must expect that
selection of spacecraft options will be determined by very different criteria
than for the cost-constrained scientific asteroid missions of the past.So one would expect deployment of
sophisticated orbiters and dockers/landers unless it were not feasible to match
the object's trajectory, in which case only a flyby might be possible.

Still more can be learned from an orbiter (for which the
NEAR Shoemaker investigations of Eros provides an inexpensive baseline:see Sept. 22, 2000, issue of Science,
Vol. 289, No. 5487).NEAR's instruments
can be augmented by carrying ground-penetrating radar sounders (most useful for
C-type asteroids); using neutron beam/gamma-ray techniques to assess atomic
composition in upper tens of cm of the surface; using laser ablation and other
approaches to assess isotopic composition, volatility, etc.; and serving as an
observation post for modules deployed to the surface (seismic net, dropped
objects, etc.).

For investigating a small asteroid, there is not a very
great jump from an orbiter to a lander/docker (even NEAR Shoemaker successfully
"landed" on Eros and kept operating, although it was not designed to
do so and lacked landing gear and "feet").There are issues about how to establish firm contact and coupling
with a small asteroid and one might expect an iterative approach to
investigating the mechanical attributes of the surface (compressibility,
strength, etc.) and making use of surface contact to study internal structure
and other attributes that cannot be determined by remote sensing.

Logistics.With no limitation on
resources, it would still take ~18-24 months to get a flyby mission ready for
launch.Arrival might take anywhere
from a few additional months to quite a few years, with flyby speeds of a few
to 20 km/sec or more.Solar electric
propulsion might become an off-the-shelf capability in the future, and could be
used to shorten durations to arrive at the target.More capability and time would generally be required to match trajectories
in order to orbit/dock with the object.Studies should be done on what simplified fall-back strategies might be
possible in the event of short notice or resource limitation.

Instead of waiting to characterize threatening objects
only after they are discovered, consideration should be given to the advantages
to building a capability in advance.For example, comets will be particularly difficult to characterize well
in the short time likely to be available.Characterization of a generic, accessible comet in advance is one
approach.Another is to deploy one or
more spacecraft in high-energy orbits in advance, properly equipped to assess a
comet when and if needed.

Deflection/Destruction
of Threatening Body

The most direct approach to mitigating an impact is to
ensure that the impact does not happen, by either deflecting or destroying the
incoming body.In a wide range of
cases, "destruction" of the body poses the potential risk of greatly augmenting
the danger.Whether or not impact with a
broken body, or parts of it, would be more dangerous than the impact of the
original, intact body, the practical problems of dealing with the numerous,
randomly deployed fragments of a disrupted body would be enormous.Nevertheless, there are some cases (e.g.
very small body) where effective destruction can be assured; it is important to
define what case/s are appropriate for considering destruction rather than
deflection.

Roughly 3 x 10-3 joules/kg are required to
change a body's velocity by ~10 cm/sec, which is what would be generally
required to change a predicted strike one year later into a miss.That is at least 1000 times less energy than
what would generally be required to disrupt bodies 0.1 to 1 km in diameter
according to our estimates, which are conservative by 1 to 2 orders of
magnitude compared with other published estimates.So there is considerable margin available for deflecting a body
rather than disrupting it, provided that the energy can be efficiently applied
to deflection.

The most controlled approach to deflection would be by
non-instantaneous (low-thrust) technologies, which don't exist today in
practical terms: solar-sail, powerful ion drives, mass drivers, etc.There are practical difficulties yet to be
resolved in how to attach such devices to a small body.Moreover, the long durations of operation
raise issues of maintenance.

If one could dock-and-push with the high impulse thrust
of a chemical rocket, a variety of cases could be dealt with.The Space Shuttle main engine could just
deflect a 1 km asteroid, given 30 years advance warning.A Delta 2 first stage, with a several-minute
burn, could deflect a 100 m object given 6 months warning.

Far more energy can be delivered to an object by a bomb,
and that is the only option available with current technology for dealing with
the largest potential impactors on shorter time scales than just
discussed.There are considerable
uncertainties about coupling the energy of a bomb into the object, and doing it
over a broad enough area to avoid disruption by the stresses of a very
localized release of the energy (a stand-off explosion of a neutron bomb
addresses this issue).If the coupling
efficiency were as low as 1%, then one starts to challenge the margin of safety
against disruption.

Another explosive approach to deflection would be by
kinetic kill -- maneuvering an object into the path of the object to be
deflected and relying on the resulting hypervelocity collision to do the
trick.Our preliminary assessment is
that such an approach may be feasible for objects <100 m in size but would
be generally impractical for 1 km bodies.

An issue illustrating the uncertainties of evaluating the
dangers of break-up is the fact that comets are sometimes observed to
spontaneously break up.It is unlikely
that such break-ups result from especially energetic phenomena on comets.They probably say more about the fragile
nature of comets than anything else.A
relevant question is whether cometary fragments, then, might still be
hazardous?

As with reconnaissance of a threatening object's nature,
one may gain precious time by deploying planetary-defense "soldiers"
in space located in orbits that may shorten the time it takes to arrive at the
threatening object.Such a strategy, of
course, involves advance commitments to very expensive hardware before a threat
has been identified.

Whatever approach is taken to interacting with a
threatening body, there are technical issues specific to impactor mitigation
that need to be addressed.For example,
there are difficulties involved with terminal navigation (e.g. how does one
assuredly acquire small, non-reflective bodies prior to arrival?).While there has been preliminary thinking
about many of these issues (cf. Gehrels, 1994; Canavan, Solem & Rather, 1993;
Gold, 1999), no thorough, systematic evaluation of an NEO deflection system has
yet been done.

Other Mitigation
Approaches

Whether or not there is an opportunity to deflect an
oncoming asteroid or comet and prevent a threatened impact from happening,
there are many additional approaches to mitigating the hazard.These range from evacuation of ground-zero
to taking advance action to prevent other social, economic, and environmental consequences
from being as bad as they might otherwise be.Decisions on what steps to take should be based on the elements that
define the Torino Scale (probability of an impact happening, magnitude of the
impact) and on the duration until the impact and the location of the impact
(when and if known).

Plans should be developed now to take the initial
steps towards various kinds of mitigation, for different scenarios based on the
matrix of possibilities described above.Even for cases of impact predictions that don't actually turn out to be true
(say a near miss), much may have to be done to calm public fear and panic
arising from the prediction itself.The
same may be true for actual impacts that experts might think are too small to
merit attention; consider the public's fear of the fall of Skylab in the 1970s
[we are presently awaiting public reaction to the impending re-entry of the Mir
space station].Even the impact of a
very small body, if the time and location are predicted with an hour's warning
or more, might merit evacuation of people from ground zero.An impact of any scale is bound to do
more damage near ground zero, or along the coasts of the body of water that is
impacted, than elsewhere, so evacuation (and other localized mitigation steps,
if there is time) can always help.

If, as would likely be the case, there is ample warning
(years or more) of a large impact, even the effects of a potential
civilization-destroyer could be minimized through proper advanced
planning.For example, food supplies
could be built up and stored safely, sufficient to carry the world's population
through a year without agriculture.Since the outlines are known of the kinds of climatological changes that
would be induced by an impact, measures could be taken to prepare for such
changes.A predicted impact that might
kill hundreds of millions of people, were they caught unawares, could be
converted into a Y2K-like non-event -- interesting but without serious damaging
consequences.

Near ground zero, and elsewhere (perhaps even worldwide,
depending on scale of impact), steps could be taken -- in addition to
evacuation -- similar to precautions taken in earthquake zones (like enhancing
the integrity of structures), in forested areas (lessening susceptibility to
fire), and in protecting infrastructure assets during space weather events
(since an impact could well induce electromagnetic disturbances).Along coasts, an understanding needs to be
developed of how to predict, recognize, and track an impact-tsunami and perhaps
prepare residents for events much larger than the tsunami events that have been
predicted during their lives.Since it
is most likely that a small impact would currently happen without advanced
warning, there should be established within existing tsunami detection and
warning systems a protocol for recognizing an impact-produced tsunami and
understanding how its effects might differ from those produced by more usual
causes (e.g. earthquakes).

Although there is little reason, in usual cases, to
expect that a major impact will be preceded or followed by other impacts, there
are several reasons why the public may expect such scenarios.First, earthquakes are generally followed by
aftershocks, which may have the potential for causing major damage (and many
other kinds of natural disasters occur over a longer period of time and/or
occur in clusters).Second, fictional
portrayals of cosmic impacts (e.g. in some blockbuster movies) have, for
dramatic reasons, included both precursor impacts and multiple impacts, raising
that probably-false possibility in the minds of many people.So consideration will have to be given to
sounding an "all clear" after any predicted impact happens.The distinction regarding radiation between
nuclear weapons and impacts also needs to be communicated; while there are
various long-lasting consequences -- some potentially hazardous -- of an
impact, exposure to radioactivity (at least due to the direct impact itself) is
not one of them.

RECOMMENDATIONS

Our primary recommendation is that much broader groups of
people need to be educated about impact hazard issues, beyond the superficial
and often incorrect impressions they may have gotten from their chief exposures
to these matters: exaggerated/retracted news stories about impact predictions
and "near misses," and movies like "Armageddon."A much broader segment of the technical
community, beyond astronomers and space engineers, needs to appreciate and
become familiar with technical aspects of this hazard.These segments include the natural hazards
community and experts in risk assessment, meteorological storms, seismicity,
climate modelling, etc.In addition,
public officials responsible for mitigation of (and response to) emergencies
and disasters need to understand the basic attributes of the impact hazard.These include the chains-of-command in the
military and in the law-enforcement/civil defense infrastructures.

Research, planning, and preparation need to commence now,
although it remains to be determined how far such activities should go, given
the low probabilities of having to address any real, major impacts in our
lifetimes.We believe that several
issues need to be addressed in the near future.

lThe notification system (concerning a
predicted potential impact) needs to be cleaned up, expanded, and officially
adopted and implemented.

lOfficial clearinghouse/s for the best
information need do be developed (potential nuclei for such functions,
including fledgling web sites or analogous capabilities, already exist at Jet
Propulsion Laboratory, NOAA, Spaceguard Foundation, and the IAU Minor Planet
Center, among others).

lSerious connections need to be developed
with the hazard mitigation community, including agencies like FEMA.

lMore objective approaches to communications
need to be developed to minimize misunder­standing of this hazard, which is so
mismatched to our personal experience base (extreme rarity or low chances of
happening vs. extreme potential consequences).In other words, the Torino Impact Hazard Scale needs to be further
developed, extended, distributed, and explained.

lOfficial international channels for
exchanging information about NEO hazard-related issues and events need to be
developed.

lWithin the United States, an interagency
approach, and assignment of responsibilities, for dealing with the NEO hazard
needs to be developed; the Global Change Program may provide a template.Analogous steps need to be developed in
other nations and to coordinate among nations.

lEducation about the NEO hazard would be
facilitated by conducting a high-visibility, international conference on the
NEO hazard, emphasizing the non-astronomical, non-NEO-deflection issues that
have so far been treated as backwater concerns in previous NEO hazard
conferences.Perhaps a newsletter
should be instituted.

lGiven widespread interest in extending the
Spaceguard search down to bodies much smaller than the 1 km goal of the U.S.
search efforts, a thorough evaluation of ground- vs space-based approaches
needs to be made.Although spacebased
efforts are usually vastly more expensive, they have advantages that may
balance the costs in some cases; in other cases, the cost of spacebased efforts
may not be relevant (e.g. the searches may be piggy-backed onto other endeavors
that pay most of the costs).

lWe consider the case of comets to be
astonishingly intractable (they are difficult to detect, there is a short time
between detection and impact so the object can't be studied carefully, a comet
may be difficult or time-consuming to get to so it may not be possible to
"blast" it until it is almost here, a comet's motion is difficult to
predict, and the structural nature of comets is poorly known -- they break-up
independently and unpredictably).So we
recommend more detailed study of the nature of comets and of cometary
detection/mitigation strategies.At a
minimum, we must quickly assess how large a part of the impact hazard comets
are.

lChemical rockets may have quite wide
applicability to deflection scenarios; we recommend more study of that
technology.

lIn certain cases of attempted mitigation,
disruption is more likely than deflection.More research needs to be done in this area, including studies of the
potential consequences of disruption.

lAll of these recommendations are predicated
on a political decision about the importance of the NEO hazard and about the
level-of-effort that should be expended in addressing it.The technical community needs to identify
potential criteria (beyond simple comparisons of death rates from various
hazards) for making this judgement.We
recommend that official, objective study/ies by bodies like the National
Research Council be done for this purpose.Ultimately, society's decision about how seriously to address the impact
hazard will have to involve broad segments of the public, beyond the technical
community.

ACKNOWLDGEMENTS

This white paper was primarily supported by a
Presidential Discretionary Internal Research and Development Grant by Southwest
Research Institute President Dan Bates.Some research reported here was also supported through NASA's NEO
Program Office at the Jet Propulsion Laboratory.We appreciate the interest in this project of Walter Huebner and
discussions with many colleagues.